The present invention relates to phantoms for x-ray machines and the like, and in particular, to a phantom providing calibration for dual energy x-ray equipment used in bone density measurement.
The absorption of x-rays by material is dependent on two independent atomic properties of the material, the Compton scattering of x-rays by the material and photoelectric absorption. The relative proportion of these two types of absorption is different for different materials and varies as a function of frequency. Accordingly, measurements of the material at two x-ray energies (generally corresponding to different x-ray frequencies) can reveal information about the properties of the material.
Dual energy measurements can be used to measure relative proportions of any two basis materials making up the material. A principal use of this phenomenon is in the measurement of the basis materials of bone and soft tissue to provide an in vivo measurement of bone mass. Lunar Corporation, the assignee of the present application, manufactures a number of dual energy x-ray machines suitable for this purpose as described in the following U.S. patents hereby incorporated by reference: U.S. Pat. Nos. 5,253,282; 5,228,068; 5,287,546; 5,291,537; 5,305,368; 5,306,306; 5,408,439; 5,485,492; 5,509,042; 5,533,080; 5,533,084; 5,577,089 and 5,673,298.
In making bone density measurements, it is important that repeatable quantitative results be produced. For this reason, it is necessary that a machine be calibrated frequently to prevent drifting of the measurements over time. In the case where multiple machines may be used in a study, each machine must be cross-calibrated to identify inconsistencies among the machines. Critical too, for bone density measurements, is that a similar region of the bone be examined among patients or for an individual patient over different scans. Finally, when making calibrations for bone density, it is important that they show the same range of readings for different samples.
What is needed is a simple and accurate phantom that can be used with dual energy densitometers to monitor its calibration performance over time, as well as to provide cross-calibration of multiple densitometers, by simulating measurement of the irregular and inhomogeneous spine.